1. Field of the Invention
The present invention relates to a solid-state image sensor and, more particularly, to an improvement in a high sensitivity solid-state image sensor of such as a CSD (Charge Sweep Device) type.
2. Description of the Prior Art
Recently, the degree of large scale integration of solid-state image sensors has considerably increased and the area occupied by each picture element has become very minute. With such an increased degree of large scale integration, it has come to be required that solid-state image sensors are of increased sensitivity. In order to meet such requirement, solid-state image sensors of a CSD type were developed. The fundamental operation of such CSD type solid-state image sensors is described in the paper entitled "A 480.times.400 Element Image Sensor with a Charge Sweep Device" by M. Kimata et al. appearing in pages 100 and 101 of ISSCC, DIGEST OF TECHNICAL PAPERS, February, 1985. Furthermore, a color image sensor employing the CSD type is described in the article entitled "A 1/2" Form at Color Image Sensor with 485.times.510 Pixels" by M. Yamawaki et al. appearing in pages 91 through 94 of Technical Digest of Electronic Imaging '85. In summary, the above described CSD type solid-state image sensor is structured such that signal electric charges read from photoelectric converting devices corresponding to one horizontal line are swept to the vicinity of the horizontal transfer device (horizontal CCD) through the vertical transfer devices (vertical CCD) during one horizontal period and the same are further transferred to the horizontal CCD during the horizontal blanking period, so that the same are read out in succession during the next horizontal period. According to such a system, even if channel width of the charge transfer means in the vertical direction (vertical CCD) is made very narrow, an increased amount of signal charges can be transferred and, accordingly, a conversion area/pixel area ratio, i.e. the ratio of the area occupied by the photoelectric converting device to the area of each pixel can be increased.
FIG. 1 is a plan view showing a solid-state image sensor of such a conventional CSD type. FIG. 2A is a sectional view taken along the line A--A in FIG. 1 and FIG. 2B is a sectional view taken along the line B--B in FIG. 1.
First, the structure of the conventional CSD type solid-state image sensor shown in FIGS. 1 to 2B will be described.
Referring to FIG. 1, one pixel of the solid-state image sensor comprises a photoelectric converting device 22 formed of a p-n junction for converting incident light into signal electric charges, a transfer gate 26 for selectively reading the signal charges from the photoelectric converting device 22, and a transfer electrode 23 or 24 for transferring the read signal charges in the vertical direction through the transfer gate 26. The electrode for the transfer gate 26 and the transfer electrode 23 or 24 for transferring the signal charges in the vertical direction are commonly implemented. A scanning line 21 constituting a signal path for selecting the photoelectric converting devices 22 constituting one row in the horizontal direction is coupled to the transfer electrode 23 or 24 through a contact hole 25.
Further referring to FIG. 2A, the structure in section taken along the line A--A in FIG. 1 is seen, wherein a transfer channel 3 of a buried type serving as an electric charge transfer path in the vertical direction is formed on a p type semiconductor substrate 1, the transfer channel 3 comprising an n.sup.- type impurity diffused layer. A gate insulating film 27 is formed on the transfer channel 3 and the transfer electrode 23 for controlling the vertical transfer operation is formed further thereon. On the other hand, the photoelectric converting device 22 is formed on the p type semiconductor substrate 1 and a thick oxide film 31 and a p.sup.+ type impurity diffused layer 32 are formed for electrical separation between the adjacent devices.
Referring to FIG. 2B, the structure in section taken along the line B--B in FIG. 1 is seen, wherein a transfer gate 26 is formed between the photoelectric converting device 22 and the transfer channel 3 and accordingly the p.sup.+ type impurity diffused layer 32 for separation of the devices is not formed therein as shown in FIG. 2A. A scanning line 21 for selecting the photoelectric converting device 22 is coupled to the transfer electrode 23 through the contact hole 25.
Now, description will be made of an outline of the operation of the conventional CSD type solid-state image sensor shown in FIGS. 1 to 2B. The operation of such solid-state image sensor is fully described in the above described publications and in an article by M. Kimata el al. appearing on page 31 of the technical report TEBS101-6ED841 of the Institute of Television Engineers of Japan.
First, photoelectric converting devices 22 in one row are selected by a single scanning line 21. The signal charges from the selected photoelectric converting device 22 are read out on the vertical directional transfer channel 3 through the transfer gate 26 and are transferred in the vertical direction. The transfer of the signal charges in the vertical direction is done during one horizontal period and the same are read out in the horizontal CCD during the horizontal blanking period. The CSD type solid-state image sensor structure described above has a great advantage that the width of the transfer channel portion can be made thin. More specifically, since the same is structured such that only the signal charges from a single photoelectric converting device are read out over the single vertical transfer device, an ample amount of transfer charges can be obtained even when the channel width is selected to be narrow. In other words, since the length of the potential well in the vertical charge transfer device (CSD) comes to be equivalent to the length of one vertical line, the area of the potential well becomes sufficiently large even in case of a narrowed channel width, whereby an ample amount of transfer charges can be provided.
Meanwhile, FIG. 3 is an graph showing a relation between he width of the buried type transfer channel and the channel potential formed therein in case where the same gate voltage is applied. As shown in FIG. 3, as the width of the transfer channel is decreased, the channel potential formed therein is accordingly decreased. The reason is considered to be that due to diffusion in the lateral direction of impurities from the p.sup.+ type impurity diffused layer 32 for channel cutting, the impurity concentration of the buried transfer channel 3 is compensated. Such a phenomenon becomes particularly noticeable in case where the width of the transfer channel 3 is decreased. As shown in FIG. 2B, the transfer channel 3 of the n.sup.- type impurity diffused layer connected to the transfer gate 26 is influenced by the p.sup.+ type impurity diffused layer 32 only from one side, whereas as shown in FIG. 2A, the other portion comes to be influenced by the p.sup.+ type impurity diffused layers 32 from both sides.
FIG. 4A is a sectional view taken along the line C--C in FIG. 1 and FIG. 4B is a view showing the state of a potential well formed therein. As shown in FIG. 2B, since the portion connected to the transfer gate 26 within the transfer electrodes 23 and 24, i.e. an approximate central portion of each transfer electrode, is only influenced by p.sup.+ type impurity diffused layer 32 from one side, a deep potential well 34 is formed in that portion as shown in FIG. 4B. Therefore, the charges trapped by the deep potential wells 34 in such center of the transfer electrode cannot contribute to transfer, resulting in a problem that the charges Q.sub.R left without being read arise to degrade the transfer efficiency.
Furthermore, in the conventional solid-state image sensor of FIG. 1, the transfer electrode 23 is formed with a first layer of polysilicon, for example, and the transfer electrode 24 is formed with a second layer of polysilicon, the electrode of the transfer gate 26 is implemented commonly to the transfer electrode 23 or 24, as described previously. This means that the electrodes of the transfer gates 26 to be common to the transfer electrodes 23 of the first layer and the same to be common to the transfer electrodes 24 of the second layer are formed to be disposed alternately. Accordingly, if and when there are increased disregistration of the mask, difference in working dimension, difference in thickness of the gate insulating film 27 between the transfer electrode 23 of the first layer and the transfer electrode 24 of the second layer, then a difference occurs in the transistor characteristic between the transfer gate 26 to be common to the transfer electrode 23 and the transfer gate 26 to be common to the transfer electrode 24. Due to such a difference in the transistor characteristics, a difference occurs in the signal charges of the photoelectric converting device 22, resulting in a problem that a so-called fixed pattern noise occurs in which a difference in output is seen alternately for each horizontal line even if a uniform image is being sensed.